1. Field of the Invention
The present invention relates to an image recording apparatus such as an inkjet recording apparatus and, in particular, to such an image recording apparatus including (a) a pair of guide frames that are separate from each other in a conveying direction in which a recording medium is conveyed, and that extend parallel to each other in a direction perpendicular to the conveying direction; (b) a carriage including a main frame that is supported by the two guide frames such that the main frame bridges the two guide frames and that is reciprocated on the two guide frames in the direction perpendicular to the conveying direction; and (c) an image recording head such as an inkjet recording head that is mounted on the main frame of the carriage.
2. Discussion of Related Art
There has been known a so-called ink-jet-type image recording device including an ink-jet recording head that ejects droplets of ink toward a recording medium such as a recording sheet and thereby records an image on the recording medium. The recording head is mounted on a carriage that is supported by one or more guide members, and is reciprocated in a direction perpendicular to a conveying direction in which the recording medium is conveyed. While the carriage is reciprocated, the recording head ejects the droplets of ink toward the recording medium. Since the recording head is mounted on the carriage supported by the guide members and the recording medium is supported by a platen, a distance (i.e., a gap) between the recording head and the recording medium is made uniform over an area in which the recording head is reciprocated.
The distance between the recording head and the recording medium influences a degree of definition of the image recorded on the recording medium. In the case where a recording sheet is used as the recording medium, a distance or gap between an ink-ejection surface of the recording head and an image-recording surface of the recording sheet changes depending upon a thickness of the recording sheet. There are known various sorts of recording sheets such as an ordinary sheet, a glossy photo-print sheet, a postcard, or an envelope, and those sheets have different thickness values. If different sorts of recording sheets are used, then the above-indicated gap changes and accordingly a quality of an image recorded on each sort of recording sheet may change, that is, an image having a desired quality may not be recorded. In addition, recently, there has been such a tendency that the size of ink droplets ejected by the recording head is decreased for the purpose of increasing the quality of images and, to this end, the gap between the recording head and the recording medium is decreased. Thus, the recording head may be interfered with by a recording medium having a great thickness. In this technical background, there has been proposed a gap adjusting device that adjusts a gap between a recording head and a recording medium by moving, depending upon a thickness of the recording medium, a recording head in a direction of thickness of the medium. This gap adjusting device is disclosed by, e.g., Patent Document 1 (i.e., Japanese Patent Application Publication No. 2003-231326), Patent Document 2 (i.e., Japanese Patent Application Publication No. 2003-341173), or Patent Document 3 (i.e., Japanese Patent Application Publication No. 2003-175654).